1. Field of the Invention
The present invention relates to a matrix switching apparatus used for input apparatus such as a keyboard, and more particularly relates to a matrix switch for preventing a pseudo input condition caused by a detour of a signal as well as an output of an erroneous signal caused by a noise intruding into an apparatus and the like.
2. Prior Art
The prior keyboard with matrix switches has been used as signal input apparatus for various electronic machines such as a typewriter. Upon using the aforementioned keyboard, however, as shown in FIG. 6, a so-called detour of a signal occurs when a plurality of key switches are operated simultaneously, resulting in pseudo input conditions in which non-operative keys are falsely detected as operative. Namely, a matrix switch circuit shown in FIG. 6 is connected to a scan-type encoder so as to output a scanning signal with level `0` to a driving line sequentially, which encodes key switches operated in accordance with detected voltage on a sensing line. The above driving and sensing lines are arranged in a matrix, respectively. When switches SW1, SW2 and SW3 are turned ON simultaneously, an electric current via a resistance R2 flows through SW3, then flows back through SW2, and further flows through SW1 into a driving line DL1. Consequently, a voltage V2 on a sensing line SL2 becomes `0` so that it seems as if a switch SW4 was ON. That is to say, assuming a square or an elongated square with four key switches in the matrix at each corner, when three switches of the square are turned ON, the remaining one is brought into a pseudo input condition.
Attempts have been made in the prior arts to solve the above problem. For example, as shown in FIG. 7, diodes D1-D6 for preventing reverse of current are connected in series with the key switches SW1-SW6 respectively in order to avoid a detour of a signal. Published unexamined Japanese Patent Application No. 60-39718 discloses a matrix switch circuit in which resistances are connected in series with key switches respectively, so that a logical circuit detects the difference between two voltages caused on a signal detection side both when a plurality of switches are operated simultaneously and when a pseudo input switch is really operated, so as to remove a detour signal.
The prior matrix switch circuit with diodes, however, fails to miniaturize and simplify the key switches, and allows its cost to be high. The circuit with resistances disclosed in the above Patent Application has defects such as an increase in the number of parts and manufacturing processes.
Moreover, the input apparatus with key switches, regardless of a contact or contactless switch types, generally have likelihood of generating erroneous function signals owing to a chattering phenomenon and an effect of the noise intruding into the apparatus, which causes an erroneous type and the like.